1. Field of the Invention
The present invention relates to air pollution control, and more particularly, to treating flue gas to reduce pollutant emission therefrom.
2. Description of Related Art
A variety of devices are known in the art for controlling pollution in exhaust and flue gas, for example, flue gas emitted from boilers. Among such devices, many are directed to reduction of NOX, CO, VOC, and the like, from flue gas prior to releasing the flue gas into the atmosphere. For years, a commonly employed technique for reducing NOX, CO, and VOC emissions was to modify the combustion process itself, e.g., by flue gas recirculation or the overfire air. However, in view of the generally poor proven results of such techniques (i.e., NOX removal efficiencies of 50% or less), recent attention has focused instead upon various flue gas denitrification processes (i.e., processes for removing nitrogen from flue gas prior to the flue gas being released into the atmosphere).
Flue gas denitrification processes are categorized into so-called “wet” methods, which utilize absorption techniques, and “dry” methods, which instead rely upon adsorption techniques, catalytic decomposition and/or catalytic reduction. At present, a widely implemented denitrification process is selective catalytic reduction (SCR), which is a “dry” denitrification method whereby the introduction of a reactant (e.g., NH3) causes reduction of the NOX, which, in turn, becomes transformed into harmless reaction products, e.g., Nitrogen and water. The reduction process in an SCR process is typified by the following chemical reactions:4NO+4NH3+O2--->4N2+6H2O2NO2+4NH3+O2--->3N2+6H2OOxidation catalysts can be used to cause oxidation of carbon monoxide (CO) and/or so-called volatile organic compounds (VOCs). An exemplary oxidation catalyst is a precious metal oxidation catalyst. CO/VOC oxidizing catalysts can operate without reagent using unreacted oxygen in the flue gas to convert CO to CO2 according to the following reaction:CO+½O2--->CO2 
Due to the technology involved in SCR, there is some flexibility in deciding where to physically site the equipment for carrying out the SCR process. In other words, the chemical reactions of the SCR process need not occur at a particular stage or locus within the overall combustion system. The two most common placement sites are within the midst of the overall system (i.e., on the “hot side” upstream from the air heater), or at the so-called “tail end” or low dust portion of the overall system (i.e., on the “cold side” downstream from the air heater).
Unfortunately, significant problems are encountered in industrial settings with respect to both hot side and cold side SCR installations. For example, hot side SCR processes are not optimal for use in conjunction with wood-fired burners. This is because ash present within the wood contains alkalis, which can cause damage to the catalyst due to poisoning during the SCR process. Cold side SCR processes avoid this disadvantage because the particulate matter is removed prior to reaching the catalyst, but are plagued by thermal inefficiency due to their reliance on indirect heat exchangers.
Use of SCR systems in biomass-fueled plants requires locating the SCR system after the particulate control device to limit the SCR catalyst's exposure to damaging compounds carried in the flue gas, such as alkaline metal (Na, K, etc.) compounds. To minimize the damage from these compounds, SCR systems in biomass-fueled plants are typically located at the ‘tail end’ of the plant where the flue gas temperature is in the range of 280° F. to 380° F. In this low temperature range SCR systems need heat input from some auxiliary source, typically from a gas-fired and/or oil-fired burner(s), to raise the temperature of the flue gas to a temperature range, typically 430° F. to 600° F., that allows sufficient SCR catalyst activity. However, the additional heat input must be recovered to minimize the impact of the SCR system on the plant's efficiency. Two approaches for an SCR system in a biomass-fueled plant have been used, as described below.
First, a conventional ‘tail end’ SCR system uses an auxiliary heat input device, such as burners or steam coils in the flue gas duct to raise the flue gas temperature prior to the SCR catalyst. A recovery heat exchanger (a recuperator) recovers only 60% to 70% of the auxiliary heat input (limited by exponentially rising costs for greater recovery) typically by transferring heat from the flue gas stream exiting the SCR to the flue gas stream before the auxiliary heat input device. In addition to its low heat recovery, a conventional ‘tail-end’ SCR system requires significant additional fan power, typically a booster fan, to overcome pressure drop through the SCR catalyst and the recovery heat exchanger.
A second alternative is regenerative SCR(RSCR) technology that integrates auxiliary heat input and heat recovery (regenerative thermal media) into a compact, modular SCR system to recover over 95% of the heat needed to raise the flue gas temperature for the SCR catalyst. RSCR technology is proven to be more cost efficient and effective NOX control technology than a conventional ‘tail end’ SCR system.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. Nonetheless, there is an ongoing need in the art for improved performance. For example, while the RSCR technology is superior to a conventional ‘tail end’ SCR, it still requires at least some auxiliary heat input. There still remains a need in the art for systems and methods that can reduce or eliminate the need for auxiliary heat input. There also remains a need in the art for such systems and methods that can reduce the pressure drop through pollution control systems. The present invention provides a solution for these problems.